1. Field of the Invention
The present invention relates to a polarization state measuring apparatus for detecting a polarization state of an input signal light by measuring the Stokes parameter or the like, and in particular relates to a polarization state measuring apparatus for achieving the reduction in measurement accuracy deterioration which occurs due to differences in effective optical path lengths.
2. Related Art
In optical communication systems, as means for increasing the transmission capacity, it is known to increase the communication speed per channel. However, in a range where a bit rate of signal light exceeds 10 Gb/s (gigabit/second) or 40 Gb/s, the pulse width of the signal light becomes several tens of ps (picosecond). Therefore, it becomes difficult to distinguish between a “0” level and a “1” level of each bit due to waveform distortion generated due to various factors. Such waveform distortion becomes a factor for determining the main specifications such as system length. Then, when designing the system, various measures are devised for arranging parts for compensating for waveform distortion.
As a factor generating waveform distortion of signal light, there is polarization mode dispersion (PMD). This PMD is a dispersion which occurs as a result that a differential group delay (DGD) occurs between two orthogonal polarization modes, for example, due to the core of an optical fiber used as an optical transmission path becoming elliptic, or due to a lateral pressure, or to a partial temperature change and the like. For example, in the case where the optical fiber is laid in a place, such as with a submarine cable, where environmental changes are minimal, or the case where the optical fiber is laid in a place where environmental changes are severe such as along the side of a railroad, the behavior of the PMD variations is remarkably different.
PMD compensators (to be referred to as PMDC, hereunder) for compensating for the abovementioned PMD have been recently developed by various companies. A configuration of a well-known PMDC is basically a loop-back system in which the waveform distortion of signal light is monitored and a compensation amount of the PMD is controlled corresponding to the monitor result. However, it is difficult to directly and quantitatively monitor a state of waveform distortion and a generated dispersion amount. As substitute means, there is typically a method for monitoring a degree of polarization (DOP). Moreover, there are also known examples measuring the bit error rate (BER), or measuring the electrical spectrum hole burning.
The DOP can be measured by using a polarization state measuring apparatus (polarimeter). As a conventional polarization state measuring apparatus, there is known an apparatus for measuring the four Stokes parameters representing a polarization state (refer to Japanese Unexamined Patent Publication No. 6-18332, Japanese Unexamined Patent Publication No. 9-72827, Japanese National Publication No. 2001-520754, and Japanese National Publication No. 2003-508772).
FIG. 6 shows a configuration of a basic optical system of a conventional polarization state measuring apparatus as described above. In this optical system, firstly an input signal light L is branched into four at 25% each, by an optical coupler (CPL) 1. Then, a first branched light passes through a ¼ wave plate (QWP) 2 and a polarizer (POL) 31 which passes therethrough only a polarized component inclined by 45° to a previously set reference plane, to be incident on a light receiving element (PD) 41. A second branched light passes through a polarizer (POL) 32 which passes therethrough only a polarized component inclined by 45° to the above reference plane, to be incident on a light receiving element (PD) 42. A third branched light passes through a polarizer (POL) 33 which passes therethrough only a polarized component parallel (or perpendicular) to the above reference plane, to be incident on a light receiving element (PD) 43. A fourth branched light is directly incident on a light receiving element (PD) 44.
If electric signals which are photoelectrically converted by the respective light receiving elements 41, 42, 43 and 44 to be output, are DQ, D45, D0, and DT, then the four Stokes parameters S0, S1, S2 and S3 are represented by the relationships shown in the following equation (1).S0=DTS1=2·D0−DTS2=2·D45−DTS3=2·DQ−DT  (1)
Here, S0 represents the intensity of input signal light, S1 represents a horizontal rectilinear polarized component (0°), S2 represents a rectilinear polarized component inclined by 45°, and S3 represents a right-handed circularly polarized component. By using the above Stokes parameters S0 to S3, the DOP to be measured is represented in accordance with the relationship of the following equation (2).
                    DOP        =                                                            S                1                2                            +                              S                2                2                            +                              S                3                2                                                          S            0                                              (        2        )            
However, in the above described conventional polarization state measuring apparatus, in order to measure the four polarized components, the input signal light passes through various optical elements such as the optical coupler, the ¼ wave plate, the polarizer and the like, to be received by the light receiving element. However, since respective optical paths up to the respective light receiving elements are different to each other, differences occur between times until the input signal lights reach the light receiving elements, with a problem that the accuracy of the measured polarization state is deteriorated.
More specifically, in order to improve the measurement accuracy in the conventional polarization state measuring apparatus as described above, and to miniaturize the apparatus, the present applicants have proposed a polarization state measuring apparatus having a configuration shown in FIG. 7 (refer to Japanese Patent Application No. 2003-375749). Describing the configuration of FIG. 7 in brief, an input signal light LIN whose polarization state is to be measured, is sequentially incident on optical couplers 1A to 1C of a three staged configuration, and is thus branched into four signal lights having mutually equal powers. The branched signal lights are respectively propagated through any one of a first optical branch path on which the ¼ wave plate 2, the polarizer 31 and the light receiving element 41 are arranged, a second optical branch path on which the polarizer 32 and the light receiving element 42 are arranged, a third optical branch path on which the polarizer 33 and the light receiving element 43 are arranged, and a fourth optical branch path on which the light receiving element 44 is arranged, in order to obtain the four Stokes parameters S0 to S3 represented by the relationship of the above described equation (1). Then, the Stokes parameters S0 to S3 are calculated based on the power of the signal light received by each of the light receiving elements 41 to 44, and thus, the polarization state of the input signal light LIN is obtained. Reference numeral 5 in FIG. 7 denotes a shielding wall which blocks a stray light generated between the optical components which are arranged adjacent to each other on the respective optical branch paths through which the branched lights reflected by the respective optical couplers 1A to 1C are propagated, from being propagated toward the light receiving element on the different optical branch path.
In the polarization state measuring apparatus of the prior invention as described above, since the lengths of the optical branch paths are different from each other depending on the number of optical components arranged on each of the optical branch paths, there occurs a deviation in timing at which the input signal light LIN is received by the respective light receiving elements 41 to 44, and for example, as shown in FIG. 8, a time difference Δt occurs in output signals of the light receiving elements 41 to 44 corresponding to the same pulse of the input signal light LIN.
If this time difference Δt is estimated specifically by giving an example, in the above configuration of FIG. 7, since the optical path length of the first optical branch path is the longest, then with this length as a reference, each of differences ΔI2-1 to ΔI4-1 of the optical path lengths of the other optical branch paths is considered. Here, for example, the following values are assumed for the differences.ΔI2-1=−8.500414 [mm]ΔI3-1=−2.909586 [mm]ΔI4-1=−16.450828 [mm]
Converting the above optical wave length differences ΔI2-1 to ΔI4-1 into time differences Δt2-1 to Δt4-1 of lights being propagated through the vacuum, the following values are obtained.Δt2-1=−2.83543×10−11 [sec]Δt3-1=−9.70533×10−12 [sec]Δt4-1=−5.48741×10−11 [sec]
If it is obtained how many data of for example 40 Gb/s signal lights are entered between the above respective time differences Δt2-1 to Δt4-1, then each of bit numbers Δn2-1 to Δn4-1 becomes as follows.Δn2-1=1.13 [bit]Δn3-1=0.39 [bit]Δn4-1=2.19 [bit]
That is, there occurs the deviation of 2 bits or above between the timing at which the input signal light of 40 Gb/s is received by the light receiving element 44 and the timing at which the input signal light of 40 Gb/s is received by the light receiving element 41. In the future, if a system corresponding for example to 160 Gb/s can be configured, then it is assumed that the deviation of 8.7 bits being four times the above deviation will occur. Consequently, in the case where, within a time difference due to such an optical path length difference, the polarization state of the signal light being propagated through a transmission path and the like is changed, and then, the polarization dispersion is changed to exceed the proof strength of the system without compensating for this polarization state change, then a reception characteristic of the signal light is deteriorated. That is to say, if the measurement accuracy of the polarization state is reduced due to the optical path length differences between the respective optical branch paths in the polarization state measuring apparatus, then depending on the relationship between the bit rate of the signal light and a change speed of the polarization state in the transmission path and the like, it becomes difficult to realize a required reception characteristic.
In order to suppress the deviation in the light receiving timing in the polarization state measuring apparatus as described above, then for example, it is considered to adjust the positions of the light receiving elements 41 to 44 for each of the optical branch paths, or to arrange a crystal with large refractive index on the required optical branch path, to thereby physically match the optical path lengths with each other. However, in this case, there is a problem in that a mounting area becomes large. Furthermore, there is also a possibility of deterioration in the measurement accuracy of the polarization state, due to an insertion loss and an increase in temperature fluctuation, or a change in the polarization state.